Chapter 3. Results and discussion
3.3 Electrical conductivity of BP and phosphorene
Likewise, the electrical conductivity of BP is influenced by exfoliation through ultra-sonication, and the improvement of its electrical conductivity was identified for phosphorene. The electrical conductivity phosphorene exfoliated for 2 h was 1.00×10–3 Scm¬1, tenfold compared to 24 h-milled BP.
The maximum conductivity of phosphorene (2.16×10–3 S cm–1) was obtained after exfoliation for 6 h since the ultrasonic cavitation led to the increment of intensity of 𝐴𝐴𝑔𝑔2 peak, according to Raman spectrum. This suggests that exfoliation process induced the further arrangement of RP atoms to BP. Hence, exfoliation process via ultra-sonication clearly represents the proof of the further phase transition and following enhancement of the electrical conductivity of BP.
Figure 12. The electrical conductivity of BP and phosphorene in terms of milling and exfoliation time. The left side and right side show the electrical conductivity of BP and phosphorene as a function of milling and exfoliation time, respectively. The 24 h-milled BP was used to fabricate phosphorene owing to the highest degree of phase transition from RP to BP.
3.4 Electro-responsive properties
3.4.1 Flow curves of the fluid
To obtain insight into the electro-responsive behaviors of BP- and phosphorene-based ER fluids as a function of milling time, electrorheological properties were measured under shear stress and electric field strength. The weight fraction of ER fluids dispersed in silicone oil is generally higher than 3 wt% [85–87]. In comparison, only 1 wt% of BP and phosphorene was dispersed in silicone oil, indicating the ball-milled BP and phosphorene exfoliated via liquid-phase exfoliation are more effective ER fluids than conventional fluids.
Figure 13 exhibits the shear stress and shear viscosity flow curves in terms of the shear rate for BP-based ER fluids depending on the milling time at a weight fraction of 1 wt% under 0.5 kV mm–1 of electric field strength.
Notably, as-prepared 24 h-milled-based ER fluid displayed the significantly improved shear stress. The yield stress of as-prepared 6 h-milled and 12-h milled BP was 0.42 and 0.36 Pa, respectively. There was an increment of the yield stress when BP was milled for 18 h or longer. The yield stress of as- prepared 18 h milled fluid was 0.62 Pa. After 24 h milling, the yield stress of
the fluid showed 2.82 Pa. However, the yield stress of as-prepared 36 h milled- fluid decreased to 0.46 Pa. This tendency of yield stress is in accordance with that of the electrical conductivity. According to the electrical conductivity as a function of milling time, the electrical conductivity represented that the highest electrical conductivity after 24 h milling and the longer milling than 24 h led to a decrease of conductivity. Additionally, the electrical conductivity may affect the relaxation time which is the one of the factor of ER performance.
The shorter relaxation time of ER fluids results to the improved ER performance because relaxation frequency of the fluid is proportional to the rate of polarization [11]. Furthermore, it has been reported that the higher conductivity can induce the faster polarization response, resulting to enhancement of shear stress [12].
The relaxation time, denoted as the interfacial polarization response of ER fluids, affects the dielectric properties and ER activity [19–21]. The relaxation time (𝜏𝜏𝑡𝑡) is given by [12,88]:
𝜏𝜏𝑡𝑡 = 2𝜋𝜋𝑓𝑓1
𝑚𝑚𝑚𝑚𝑚𝑚 (1)
where 𝑓𝑓𝑚𝑚𝑚𝑚𝑚𝑚 is the maximum peak of the dielectric loss factor. The relaxation time of 24 h milled-BP sheets was #, which is faster than others. Thus, 24 h- milled BP-based ER fluid exhibits the faster polarization and possesses
stronger sheet polarizability. These results indicates that 24 h-milled BP was the optimum condition in shear stress and shear viscosity flow curves.
Figure 14a exhibits the flow curves as a function of the shear rate for the 24 h-milled BP fluid under 0.01 kV mm-1. The weight fraction of previous studies of the ER fluids utilized the dispersants with their concentration higher than 5 wt% in medium. And, they showed the ER behavior above 1 kV mm-1. In comparison to them, the 24h-milled BP fluid displays ER behavior at a low concentration of dispersant (1 wt%) and small electric field (0.01 kV mm-1 ) compared to the conventional ER fluids. This indicates that BP-based ER fluids possesses the lower power consumption.
Figure 14 b represents the electro-responsive properties under various electric field strength. Without an electric field, the shear stress of the ER fluid shows a typical Newtonian fluid behavior where the shear stress linearly increases in proportional to the shear rate [89]. Under electric field, 24 h- milled BP-based ER fluid exhibits shear stress curve. In the low shear rate region, the ER fluid shows typical Bingham plastic behavior owing to the formation of chain-like structures induced electrostatic interactions between nanosheets [90,91] Bingham plastic behavior is described by the following equation;
𝜏𝜏 =𝜏𝜏0+𝜂𝜂0𝛾𝛾̇ for 𝜏𝜏 ≥ 𝜏𝜏0 (2)
𝛾𝛾̇ = 0 for 𝜏𝜏 <𝜏𝜏0 (3)
where τ denotes shear stress, 𝜏𝜏0 is the yield stress, 𝛾𝛾̇ is the shear rate, and 𝜂𝜂0 is viscosity [92]. In this behavior, the plateau region is observed. The occurrence of the plateau region is attributed to the balance between electrostatic interactions caused by interfacial polarization of sheets and hydrodynamic forces from shear flow [7,92]. However, the balance between two forces begins to destroy at the critical shear rate. In the high shear rate, the deformation rate increases faster than the reformation rate with increasing the shear rate; the hydrodynamic forces are dominant than electrostatic forces.
This is due to the insufficient time to recover as the shear rate increases [7].
Consequently, this phenomenon leads to collapse the chain-like structure and it shows the Newtonian fluid behavior.
The correlation between shear stress and applied electric field strength was also investigated under fixed shear rate. The shear stress increases in proportion to the electric filed strength. When applied electric field strength increases, the more rigid, highly linked chain-like structures form due to the stronger electrostatic interactions between BP sheets.
Figure 13. Shear stress and viscosity as a function of milling time for 1 wt%
BP in silicone oil under 0.5 kV mm–1.
Figure 14. Shear stress and viscosity for 1 wt% BP in silicone oil under a) 0.01 kV mm–1 and b) various electric strength fields (0–0.5 kV mm–1).
3.4.2 The effect of morphology on ER activity
To examine the effect of morphology on ER performance, as-prepared 24 h milled BP was utilized due to the highest intensity of 𝐴𝐴𝑔𝑔2 peak and the lowest shoulder peak of 𝐴𝐴𝑔𝑔1. The yield stress of phosphorene exfoliated for 2 h-based ER fluid is observed at 5.8 Pa, higher than the yield stress of phosphorene exfoliated for 6 h-based ER fluid (0.8 Pa). This is owing to the effect of morphology of the phosphorene nanosheets. In terms of the phosphorene-based ER fluid behavior, a sheet-like morphology enhances the ER activity compared to a spherical shape due to the higher stress applied unit sheet area [23]. The morphology effect of the ER fluids affects the flow resistance and the mechanical stability [4]. The flow resistance between phosphorene nanosheets and insulating medium increases with increasing the aspect ratio of the fluids, indicating that the flow resistance of sheet is higher than spherical form. The relation between flow resistance and aspect ratio can be determined according to the dynamic drag force (𝐹𝐹𝑑𝑑):
𝐹𝐹𝑑𝑑 = 12𝜌𝜌𝐶𝐶𝑑𝑑𝐴𝐴𝑣𝑣2 (4)
where ρ denotes the density of the fluid and A is the cross-sectional area and v is the relative velocity, and Cd is the drag coefficient, which depends on the morphology. Hence, the sheet-type with higher drag coefficient induces the stronger steric hindrance to the shear force, leading to a larger stress [4].
Furthermore, it is known that the ER fluids with a high respect ratio can link together more easily under electric field, generating the more rigid chain-like structure due to the increased shear stress [95]. Accordingly, the morphology change by exfoliation process depending on time contributes to the ER performance.
Figure 15. Shear stress and shear viscosity of 24 h-milled BP, phosphorene exfoliated for 2 h and 6 h-based ER fluids (1 wt% in silicone oil under 0.5 kV/mm of electric field).
Chapter 4. Conclusion
This study provides the facile method to fabricate the BP through mechanochemical ball mill process and phosphorene via liquid-phase exfoliation with ultra-sound. The ball mill procedure was utilized for phase transition from RP to BP as a function of milling duration at ambient condition, and ultra-sonication was employed to exfoliate as-synthesized BP in solvent by controlling exfoliation time. These method revealed the simple, massive and environmentally stable productions. In addition, as-synthesized BP and phosphorene applied the ER performance as the ER fluid on the improved shear stress and viscosity. 24 h-milled BP exhibited the outstanding shear stress (2.82 Pa) than other samples since 24 h-milled BP possessed the highest the degree of phase transition and electrical conductivity (1.04×10–4 S cm–1).
Phosphorene exhibited the higher shear stress than BP (5.8 Pa). Exfoliation procedure increased the degree of phase transition from RP to BP and the electrical conductivity to 10-3 S cm–1. Furthermore, exfoliation process induced the morphology changes, which led to 0an increase in electrical conductivity. This suggests that BP and phosphorene can offer the advanced methodology for the state-of-the-art ER activity and enhance its ER performance by controlling the degree of phase transition, morphology, and electrical conductivity.
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국문초록
지능형 소재는 빛, pH, 전기, 자기장, 온도, 기계적 응력과 같은 외부 자극에 의해 반응하여 구조와 특성을 변화시키는 물질로써 다양한 분야에서 많은 관심을 받아왔다. 지능형 소재 중, 전기장에 의해 반응하는 지능형 소재는 빠른 응답속도, 간단한 유체 역학, 가역적 거동 그리고 낮은 작동 전력으로 인해 응용 가능성이 높은 소재이다. 그러므로, 이를 실제로 응용하기 위해서는 가격이 저렴 하면서 적은 양으로도 전기 유변학적 거동을 보이는 물질이 필수 적이다. 따라서, 이 논문에서는 상대적으로 유전율이 높은 흑린 (BP)과 흑린을 박리하여 형성된 포스포린 (phosphorene) 나노 시트를 제조하여 이를 전기 유변학적 유체에 적용하였다.
상온·상압 조건에서 적린 (RP)에 볼밀 공정을 이용하여 흑린을 제조하였고, 초음파를 이용한 액체 상 박리를 통해 흑린의 2차원 물질인 포스포린을 제조하였다. 제조된 흑린과 포스포린 나노 시트 는 전기 유변학적 유체에 적용되었으며, 볼밀 시간과 박리 시간 조 절을 통해 상전이 정도, 형태학적 및 전기전도도 변화를 유도하여 전기유변학적 성능을 향상시켰다. 특히 적린을 24 시간 밀링을 진 행하였을 때, 적린에서 흑린으로 상전이가 가장 많이 유도되어 가 장 높은 전단 응력을 보였다. 또한, 24 시간 밀링하여 제조한 흑린